1. Field of the Invention
The present invention relates to a method of and apparatus for compressing and encoding digitized moving picture signals and, more particularly to a moving picture coding apparatus that is suitably used for transmitting and storing digitized moving picture signals such as television signal, and a method of coding the signals.
2. Description of the Background Art
As a prior art, a moving picture coding apparatus for compressing and encoding moving picture signals such as television signals, described in "Data Compression and Digital Modulation, 1995", Nikkei Electronics Books, published by Nikkei Business Publications, Inc., on page 58, FIG. 11. What is described is a moving picture coding apparatus using an inter-frame coding system for encoding the moving picture signal after removing any redundancy between frames.
Such a moving picture coding apparatus comprises, for example, an intra-frame coding circuit for performing an intra-frame coding of a moving picture signal; a memory for storing an encoded signal; a decoding circuit for performing an inverse transform of the encoded signal delayed by one frame period by the memory; a vector detecting circuit for referencing the moving picture signal of the precedent frame encoded by the coding circuit to detect a motion vector of a moving picture signal of the present frame inputted thereto; a motion compensation circuit for performing a motion compensation for the moving picture signal inputted thereto using the detected motion vector; an inter-frame coding circuit for coding a differential value between the moving picture signal of the present frame subjected to the motion compensation and the moving picture inverse-transformed by the decoding circuit; and a mixer for adding the encoded motion vector to an encoded signal subjected to the inter-frame coding.
In this moving picture coding apparatus, a moving picture signal inputted to its input terminal is sequentially supplied to the intra-frame coding circuit, the motion vector detecting circuit and the motion compensation circuit. The moving picture signal supplied to the intra-frame coding circuit is transformed into the coefficients of frequency components for each block composed of a predetermined number of pixels according to a transform scheme such as a discrete cosine transform (hereinafter referred to as a DCT) and encoded within a frame. The encoded moving picture signal is sequentially stored in the memory, and supplied to the coding circuit with a delay of one frame period. The encoded signal supplied to the coding circuit is subjected to, for example, an inverse DCT (IDCT) which is an operation opposite to that performed by the coding circuit. This signal is then supplied to the motion vector detecting circuit and to the inter-frame circuit as an original moving picture signal of a preceding frame.
The motion vector detecting circuit which has received the moving picture signal of the present and a preceding frame, calculates a motion vector expressing the magnitude and the direction of a motion of a moving portion from the two moving picture signals for each corresponding block comprising a predetermined number of pixels, and supplies the motion vector to the motion compensation circuit and to the mixer. In the motion compensation circuit, the moving picture signal inputted thereto undergoes a motion compensation for each block based on the motion vector, and the moving picture signal having been subjected to the motion compensation is supplied to the inter-frame coding circuit.
In the inter-frame coding circuit, a difference between the moving picture signal of the present frame having been subjected to the motion compensation and the moving picture signal of the previous frame supplied from the coding circuit is obtained, and the moving picture signal expressed in the form of this difference is in turn encoded for each block using the DCT, in a manner similar to that performed in the intra-frame coding circuit. The encoded signal having been subjected to the inter-frame coding is supplied to the mixer, and outputted in the form of a predetermined encoded bit stream along with the encoded motion vector.
According to the foregoing moving picture coding apparatus, by referencing the moving picture signal of the previous frame which is once encoded and then decoded, the motion vector between this moving picture signal and the moving picture signal of the present frame is detected, and the moving picture signal expressed in the form of the difference between the moving picture signal of the present frame, which has been subjected to the motion compensation, and the decoded moving picture signal of the previous frame is subjected to the inter-frame coding. Thus, the moving picture signal is compressed and encoded.
However, in the foregoing conventional technique, by referencing the moving picture signal of the previous frame which has been once encoded and then decoded, the motion vector of the present frame is obtained, and the motion compensation and the inter-frame coding are carried out, based on the motion vector. Therefore, when the moving picture signal of the previous frame has an operational error, the precision of a motion vector detection deteriorates, resulting in a reduction in coding efficiency.
For example, in the case where an irreversible coding is used like the DCT at the time of the intra-frame coding, the moving picture signals obtained by decoding are not restored to the original ones. For this reason, even when there is no motion in the image, such an operational error may erroneously be detected as motion, so that the moving picture signals are subjected to motion compensation by means of their motion vectors, resulting in a failure of the moving picture signals to be subjected to an accurate positional correction. Therefore, an inter-frame coding using the signals that have been subjected to the positional correction may result in a deteriorated coding efficiency.
Moreover, since an inter-frame difference value between moving picture signals before being coded and moving picture signals after being decoded does not become zero, coded signals occur even in a static image area such as the background of the picture, so that a deterioration in image quality is caused.